Steel material for automobile chassis parts superior in fatigue characteristics and method of production of automobile chassis parts using the same

ABSTRACT

The present invention provides a steel material for automobile chassis parts which has high fatigue characteristics, does not require much cost for heat treatment, and further is superior in shapeability and a method of production of automobile chassis parts using this steel material, that is, one being a steel material to which Nb and Mo have been compositely added and having a difference 50 to 150 points between a Vicker&#39;s hardness of the center of plate thickness and a maximum value of Vicker&#39;s hardness within 0.5 mm from the surface after bending by a bending R of the plate outer surface of 2 to 5 times the plate thickness. The surface is high in hardness and the center part is low in hardness, so the fatigue characteristics and shapeability are superior. Note that if annealing under conditions giving a tempering parameter λ defined by λ=T(20+log(t)) of 14000 to 19000 (where T is the absolute temperature, t is the time (h), and the temperature rise is 660° C.), it is possible to relieve the internal stress and further improve the fatigue characteristics.

TECHNICAL FIELD

The present invention relates to a steel material for automobile chassisparts superior in fatigue characteristics and a method of production ofautomobile chassis parts using the same.

BACKGROUND ART

The axle beam arranged between left and right wheels of an automobile,the surrounding suspension members, and other automobile chassis partsare repeatedly subjected to an impact load, torsion load, etc. duringvehicle operation, so a high strength and also high fatiguecharacteristics are required. For example, Japanese Patent Publication(A) No. 2001-321846 discloses a hollow structure axle beam obtained bypress forming high strength steel pipe into an irregular cross-sectionalshape.

The axle beam of this Japanese Patent Publication (A) No. 2001-321846 isimproved in fatigue characteristics by press forming high strength steelpipe into an irregular cross-sectional shape, then heating it to a hightemperature and then rapidly water cooling it for quenching. For this,however, it is necessary to heat this to a high temperature of thetransformation point of the steel material or more, so the heating costrises and, on top of this, control of the atmosphere is required forpreventing oxidation scale or a descaling step is required, so there wasthe problem that the cost became further higher. Further, in general,high strength steel pipe is poor in shapeability and easily varies indimensions, so there was the problem that the work efficiency inassembly into the chassis fell.

Note that in the process of production of chassis parts of irregularcross-sections such as an axle beam, the material, that is, the steelpipe, is subjected to strong bending, so the bent parts suffer fromlarge residual stress. Therefore, in the past, when not quenching thematerial, the residual stress resulted in lower fatigue characteristics.Further, if performing stress-relief annealing to remove the residualstress, there was the problem that the steel material ended upsoftening, so again the necessary fatigue characteristics could not besecured.

DISCLOSURE OF THE INVENTION

An object of the present invention is to solve the above conventionalproblems and provide a steel material for automobile chassis partssuperior in fatigue characteristics which has high fatiguecharacteristics regardless of the shapeability being excellent andfurther not requiring much cost for heat treatment and a method ofproduction of automobile chassis parts using the same.

The steel material for automobile chassis parts superior in fatiguecharacteristics of claim 1 made to solve the above problems ischaracterized by being a steel material to which Nb and Mo have beencompositely added and having a difference 50 to 150 points between aVicker's hardness of the center of plate thickness and a maximum valueof Vicker's hardness within 0.5 mm from the surface after bending by abending R of the plate outer surface of 2 to 5 times the platethickness.

Further, the steel material for automobile chassis parts superior infatigue characteristics of claim 2 made for solving the same problems ischaracterized by being a steel material to which Nb and Mo have beencompositely added and having a difference 50 to 150 points between aVicker's hardness of the center of plate thickness and a maximum valueof Vicker's hardness within 0.5 mm from the surface after bending by abending R of the plate outer surface of 2 to 5 times the platethickness, then annealing under conditions giving a tempering parameterλ, defined by λ=T(20+log(t)), of 14000 to 19000 (where T is an absolutetemperature, t is a time (h), and the temperature rise is 660° C.).

The aspect of the invention of claim 3 limits the composition of thesteel material for automobile chassis parts set forth in claim 1 or 2and is characterized by having a composition of, by mass %, C: 0.05 to0.23%, Si: 0.05 to 1.0%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.01% orless, Nb: 0.01 to 0.1%, Mo: 0.1 to 0.5%, Sol. Al: 0.01 to 0.05%, N:0.006% or less, and the balance of Fe.

The aspect of the invention of claim 4 is characterized by comprisingthe composition of the steel material as set forth in claim 3 furtherincluding at least one of Ti: 0.005 to 0.03%, V: 0.005 to 0.1%, Cr: 0.1to 0.5%, Cu: 0.001 to 0.5%, Ni: 0.001 to 0.5%, B: 0.0001 to 0.003%, Ca:0.0001 to 0.003%, and Mg: 0.0001 to 0.004%.

Claim 5 and on are aspects of an invention of a method of production ofan automobile chassis part superior in fatigue characteristics aftershaping and annealing. The aspect of the invention of claim 5 ischaracterized by bending a steel material having a composition of, bymass %, C: 0.05 to 0.23%, Si: 0.05 to 1.0%, Mn: 0.3 to 2.0%, P: 0.03% orless, S: 0.01% or less, Nb: 0.01 to 0.1%, Mo: 0.1 to 0.5%, Sol. Al: 0.01to 0.05%, N: 0.006% or less, and the balance of Fe to give a bending Rof the outer surface of the plate of 2 to 5 times the plate thicknessand making the difference between the Vicker's hardness of the platethickness center and the maximum value of the Vicker's hardness within0.5-mm from the surface 50 to 150 points.

Further, the aspect of the invention of claim 6 is characterized bybending a steel material having a composition of, by mass %, C: 0.05 to0.23%, Si: 0.05 to 1.0%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.01% orless, Nb: 0.01 to 0.1%, Mo: 0.1 to 0.5%, Sol. Al: 0.01 to 0.05%, N:0.006% or less, and the balance of Fe to give a bending R of the plateouter surface of 2 to 5 times the plate thickness, then annealing itunder conditions giving a tempering parameter λ defined byλ=T(20+log(t)) of 14000 to 19000 (where T is an absolute temperature, tis a time (h), and the temperature rise is 660° C.), and making thedifference between the Vicker's hardness of the plate thickness centerand the maximum value of the Vicker's hardness within 0.5 mm from thesurface 50 to 150 points.

The aspects of the invention of claim 7 and claim 8 are characterized byfurther including in the compositions of steel materials of claims 5 and6 one or more of Ti: 0.005 to 0.03%, V: 0.005 to 0.1%, Cr: 0.1 to 0.5%,Cu: 0.001 to 0.5%, Ni: 0.001 to 0.5%, B: 0.0001 to 0.003%, Ca: 0.0001 to0.003%, and Mg: 0.0001 to 0.004%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an axle beam of an embodiment.

FIG. 2 is a cross-sectional view along the line A-A, B-B of the axlebeam of the embodiment of FIG. 1.

FIG. 3 is a graph showing the change in hardness in the plate thicknessdirection of the steel material of the present invention.

FIG. 4 is a graph showing the change in hardness in the plate thicknessdirection of a general steel material.

FIG. 5 is an explanatory view showing the bending by a 3-point bendingpress in Example 1.

FIG. 6 is an explanatory view showing the state of a fatigue test inExample 1.

FIG. 7 is a graph showing the Vicker's hardness in the plate thicknessdirection of 0.03% Nb-0.3% Mo steel in Example 3.

FIG. 8 is a graph showing the Vicker's hardness in the plate thicknessdirection of 0.05% Nb-0.05% V steel in Example 3

BEST MODE FOR CARRYING OUT THE INVENTION

The steel material for automobile chassis parts of the present inventionis a precipitation hardening type of steel material to which Nb and Moare compositely added, specifically, one having a composition of, bymass %, C: 0.05 to 0.23%, Si: 0.05 to 1.0%, Mn: 0.3 to 2.0%, P: 0.03% orless, S: 0.01% or less, Nb: 0.01 to 0.1%, Mo: 0.1 to 0.5%, Sol. Al: 0.01to 0.05%, N: 0.006% or less, and the balance of Fe.

Nb and Mo are important elements for causing work hardening andimproving the strength and hardness without causing the metal structureof the surface layer to coarsen when bending giving a bending R of theplate outer surface of 2 to 5 times the plate thickness and, whenannealed, precipitating as (Nb,Mo)C upon heating to improve the strengthand hardness. To enable this effect to be manifested, Nb has to be addedin an amount of 0.01% or more and Mo in 0.1% or more. Nb has aremarkable effect when added in a small amount and is an expensiveelement, so due to the cost restrictions, the upper limit was made 0.1%.Mo suffers from similar cost restrictions as Nb and also results indeterioration of the workability if added in a large amount, so theupper limit was made 0.5%.

C was made 0.05% or more to obtain a high strength, but if over 0.23%,the toughness falls and the fatigue characteristics are affected, so thecontent was made 0.05 to 0.23% in range. Si is effective as adeoxidizing element if included in an amount of 0.05% or more, but ifexcessively added, formation of defects due to SiO₂ is invited at thetime of seam welding, so the content was made 0.05 to 1.0% in range. Mnalso has to be included in an amount of 0.3% or more to obtain a highstrength, but if over 2.0%, defects due to MnO are invited, so thecontent was made 0.3 to 2.0% in range. P: 0.03% or less and S: 0.01% orless are similar to the ingredients of usual seam-welded steel pipe.

Sol. Al and N are elements for forming AlN and promoting the increasedfineness of the crystals. If Al is less than 0.01%, the effect isinsufficient, while even if Al is over 0.05% or N is over 0.006%, acommensurate effect cannot be obtained, so the ranges were made similarto those of ingredients of usual seam-welded steel pipe.

Note that by further selectively adding to the above basic steelcomposition Ti: 0.005 to 0.03%, V: 0.005 to 0.1%, Cr: 0.1 to 0.5%, Cu:0.001 to 0.5%, Ni: 0.001 to 0.5%, B: 0.0001 to 0.003%, Ca: 0.0001 to0.003%, and Mg: 0.0001 to 0.004%, more preferable characteristics can beobtained.

Ti is an element for improving the toughness, while V and Cr areelements for assisting suppression of softening due to annealing. Cu isan element for improving strength, while Ni is an element for improvingtoughness. B is an element for improving strength, while Ca is anelement for controlling the form of the oxides and suppressing theformation of MnS. These properties are exhibited in the above ranges setfor the different elements. If below the lower limits, the effects arenot manifested, while even if over the upper limits, the effects aresaturated. Note that Mg is an element for increasing the fineness of thecrystal grains and improving the bendability and for suppressing theformation of MnS and improving the toughness of the seam part. For thisreason, 0.0001% or more is required, but inclusion over 0.004% is noteasy.

In the present invention, a steel material of the above composition isused to produce steel pipe. This is press formed to give across-sectional shape as shown in for example FIG. 1 and FIG. 2 andproduce an axle beam or other automobile chassis part. The steel pipe isproduced by the usually seam welding. The axle beam of this embodiment,as illustrated, has a cross-sectional shape close to a circle at the twoends, but is strongly bent at the center giving a bending R of the plateouter surface of 2 to 5 times the plate thickness. This type of bendingcauses work hardening, but while the surface layer rises in hardness,the rise in hardness near the plate center thickness forming the neutralaxis at the time of bending is only slight. Note that bending giving abending R of the outer surface of the plate of less than 2 times theplate thickness is difficult, while if over 5 times, the rise inhardness due to the work hardening becomes insufficient, so bendinggiving a bending R of the outer surface of the plate of 2 to 5 times theplate thickness is essential.

FIG. 3 is a view schematically showing the hardness in the platethickness direction in the steel material for automobile chassis partsof the present invention. In the state of the material, that is, theseam-welded steel pipe, the surface hardness is of a level shown by theone-dot chain line. However, due to the above bending, as shown by thebroken line, the hardness of the surface layer greatly rises. In thisway, the steel material of the present invention has large workhardening. At this stage, a large difference in hardness occurs betweenthe surface layer part and the plate thickness center part. Further, inthe steel material of the present invention, coarsening of the crystalstructure of the surface layer where work hardening occurs can besuppressed and a denser structure can be maintained.

For this reason, automobile chassis parts produced using the steelmaterial for automobile chassis parts of the present invention, as shownby the data of the later explained examples, are superior in fatiguecharacteristics and free from cracks formed from the surface.

Further, to ease the internal residual stress accompanying working andfurther improve the fatigue characteristics, the steel material forautomobile chassis parts of the present invention may also be annealed.The annealing conditions in this case can be expressed by a temperingparameter λ defined in the industry by the known λ=T(20+log(t)). Thesteel material for automobile chassis parts of the present invention canbe eased in internal residual stress by annealing under conditions whereλ becomes 14000 to 19000 (where T is the absolute temperature, tindicates the time (h), and the temperature rise is 660° C.). Note thatif below the above range, the material gradually approaches the aspectof the invention of claim 1. It becomes insufficient in terms of easingof the residual stress and the meaning of the annealing drops.Conversely, if λ is above the above range, the strength and hardnessfall and the fatigue characteristics deteriorate. Further, the heatingcost also increases, so such annealing should be avoided.

FIG. 4 is a schematic view similar to FIG. 3 for a general steel pipe.In the case of a general steel pipe, compared with the steel material ofthe present invention, the work hardening is also very slight. Further,it is shown that due to the annealing, the surface layer also softensand the majority of the hardness accompanying the work hardening islost. As opposed to this, the steel material for automobile chassisparts of the present invention is a precipitation hardening type ofsteel material to which Nb and Mo are compositely added. The workhardening is large and the heating at the time of annealing causesprecipitation of (Nb,Mo)C to improve strength and hardness, so as shownby the solid line in FIG. 3, there is almost no change in hardness alongwith annealing and the difference in hardness between the surface layerpart and the plate thickness center part due to the bending work ismaintained almost as it is.

As a result, the steel material of the present invention, both in thecase of no annealing and in the case of annealing, has a differencebetween the Vicker's hardness of the plate thickness center and themaximum value of the Vicker's hardness within 0.5 mm from the surface of50 to 150 points. Specifically, the Vicker's hardness of the platethickness center is about 200 to 250, while the Vicker's hardness within0.5 mm from the surface is 300 to 350 or so.

As explained above, in the steel material for automobile chassis partsof the present invention and the automobile chassis parts produced bythe method of the present invention using a steel material, in the statewhere the necessary bending is performed, the center part is low inhardness so a superior shapeability is maintained, while the surfacelayer part has a high hardness, dense structure and exhibits superiorfatigue characteristics. Further, by annealing the material to relieveinternal stress, it is possible to exhibit further superior fatiguecharacteristics. Note that an Nb—Ti-based steel material suffers fromcoarsening of the crystal grains at the surface layer part at the timeof annealing and cannot be given a large hardness difference like in thepresent invention. Further, cracks are liable to be formed from thesurface layer part. Therefore, it is not preferable. Below, examples ofthe present invention will be shown.

In the above embodiments, an axle beam was shown as a typical example ofan automobile chassis part, but the invention can be broadly applied tosuspension parts and other chassis parts where fatigue characteristicsare required of course.

Example 1

Steel of each of the ingredients shown in Table 1 was formed into 30 kgsteel ingots by a vacuum melting furnace. Next, each ingot was hotrolled to a plate thickness of 4.5 mm. The obtained steel plate was bentby a bending R shown in Table 1 by a 3-point bending press such as shownin FIG. 5 to obtain U-shaped fatigue test pieces. Some of the sampleswere annealed by stress-relief annealing at 580° C. for 30 min.

One end of each obtained sample was fixed to a table as shown in FIG. 6and the other end was repeatedly pushed down for a fatigue test. Samplescracking at less than 50,000 cycles were indicated as “Poor”, samplescracking 50,000 to less than 100,000 cycles as “Good”, and samples notcracking at 100,000 cycles as “Very good” in Table 2. Further, thehardnesses at the plate thickness center and a position 0.4 mm from thebending surface were measured by a load 100 gf micro Vickers system. Thedifference ΔH is indicated in Table 2 as well.

TABLE 1 Chemical Composition of Tested Steel (mass %) No. C Si Mn P S NbMo Sol. Al N 1 0.06 0.05 0.5 0.018 0.005 0.07 0.2 0.02 0.003 2 0.09 0.151.6 0.015 0.003 0.03 0.3 0.03 0.002 3 0.20 0.25 1.4 0.012 0.002 0.03 0.30.03 0.002 4 0.20 0.25 1.4 0.012 0.002 0.03 0.3 0.03 0.002 5 0.03 0.251.6 0.013 0.004 0.02 0.2 0.03 0.003 6 0.25 0.30 1.8 0.015 0.005 0.04 0.40.02 0.004 7 0.08 0.25 1.6 0.012 0.003 0   0.2 0.03 0.004 8 0.12 0.201.3 0.013 0.004 0.04 0   0.03 0.002 9 0.06 0.25 1.5 0.011 0.001 0.02 0.15 0.04 0.003

TABLE 2 Bending R of Plate Outer Surface, Presence of Annealing, Resultsof Fatigue Test, and ΔHv of Test Piece Fatigue No. Bending R Annealingtest ΔHv 1 2.5 None Good 140  2 4.0 None Good 65 3 2.5 None Good 115  42.5 Yes Very good 110  5 4.0 None Poor 40 6 2.5 None Cracks 7 4.0 NonePoor 30 8 2.5 Yes Poor 40 9 6.0 Yes Poor 35

As shown in Table 2, No. 1 to No. 4 had sufficiently large ΔHv's andalso had good fatigue test results. In particular, No. 4 gave superiorfatigue characteristics due to suitable annealing. No. 5 had a low C anda low hardness of the steel plate surface, so had a small ΔHv. No. 6 hada high C and could not be bent. This is believed to be because therewere many carbides and the bending characteristics were degraded. No. 7had a low Nb and No. 8 had a low Mo, so had a low ΔHv. This is believedbecause the steel plate surface became coarser in structure andsoftened, the surface hardness after bending was low, and the fatiguecharacteristics were also low. No. 9 had a gentle bending R and did nothave a sufficiently high surface hardness, so was believed low infatigue characteristics as well.

Example 2

A steel material of 0.16% C-0.2% Si-1.3% Mn-0.04% Nb-0.35% Mo- and thebalance of Fe was hot rolled to steel plate of a plate thickness of 3.0mm, then was formed into a φ75.0 seam welded steel pipe and was pressformed to give a bending R of the outer surface of 10 mm and annealedunder different annealing conditions. The obtained samples were measuredfor residual stress by the X-ray method, Further the hardness at theplate thickness center in the cross-section and the hardness of aposition 0.4 mm from the surface were measured by a micro Vickers system(load 100 gf) and the difference made ΔH. Further, a torsion fatiguetest was run. Samples cracking at less than 50,000 cycles were indicatedas “Poor”, at 50,000 to less than 100,000 cycles as “Good”, and 100,000cycles or more as “Very good”. The results are shown in Table 3.

TABLE 3 Annealing Tempering Fatigue tempera- Holding parameter Residualtest No. ture time (λ) stress ΔH results 1 No annealing 480 MPa  90 Good2 425° C.  15 min 13543 255 MPa  85 Good 3 500° C.  5 min 14629 95 MPa80 Very good 4 620° C. 240 min 18401 40 MPa 70 Very good 5 640° C. 720min 19248 15 MPa 35 Poor 6 700° C.  2 min 18026 35 MPa 75 Largedeformation

No. 1 and No. 2 had high residual stress, but had sufficient goodfatigue test results. No. 3 and No. 4 had low residual stress and highsurface hardness, so had extremely good fatigue test results. No. 5 hasa large tempering parameter and a low residual stress, but also has alow surface hardness. As a result, fatigue cracks are believed to occurearlier. No. 7 has a high annealing temperature and a large deformationbefore and after the heat treatment, so is judged not able to be usedfor automobile chassis parts.

Example 3

For (a) 0.03% Nb-0.3% Mo steel and (b) 0.05% Nb-0.05% V steel, steel ofa plate thickness of 3.4 mm both bent by a bending outer surface radiusof 14 mm and steel bent, then annealed were measured for distribution ofhardness in the cross-section of the plate thickness direction. Theresults are shown in FIG. 7 and FIG. 8. The (a) 0.03% Nb-0.3% Mo steelof the present invention, as shown in FIG. 7, had an extremely highhardness of the surface layer, had a large difference in hardness withthe plate thickness center part, and almost no drop in annealing. Asopposed to this, the (b) 0.05% Nb-0.05% V steel, as shown in FIG. 8, hada hardness of the plate thickness center part equal to the steel of thepresent invention, but had a low hardness of the surface layer part andhad a large drop in hardness due to annealing as well.

INDUSTRIAL APPLICABILITY

The steel material for automobile chassis parts of the present inventionis a steel material to which Nb and Mo have been compositely added.Since the work hardening is large, when bending it with a bending R ofthe plate outer surface of 2 to 5 times the plate thickness, thehardness near the surface, where the amount of deformation is greaterthan the plate thickness center, greatly rises. For this reason, thedifference between the Vicker's hardness of the plate thickness centerand the maximum value of the Vicker's hardness within 0.5 mm from thesurface can be enlarged to 50 to 150 points and, as shown by the data ofthe later explained examples, the fatigue characteristics after shapingand annealing become superior. Further, the center layer has a lowhardness, so the workability is good and shaping is possible with a highdimensional accuracy, so the work efficiency of assembly into thechassis is also superior.

In this way, the steel material for automobile chassis parts of thepresent invention exhibits superior characteristics even withoutannealing, but does not change in hardness even if annealed to eliminateresidual strain. That is, by the precipitation of (Nb,Mo)C due to theheating at the time of annealing of this steel material, the drop inhardness near the surface can be prevented and, even with annealing, thehigh hardness of before annealing can be maintained. For this reason,the difference between the Vicker's hardness of the center of platethickness and a maximum value of Vicker's hardness within 0.5 mm fromthe surface after the above bending, then annealing under conditionsgiving a tempering parameter λ, defined by λ=T(20+log(t)), of 14000 to19000 (where T is an absolute temperature, t is a time (h), and thetemperature rise is 660° C.) can be made a large 50 to 150 points.

Therefore, the automobile axle beam obtained according to the presentinvention has a high surface hardness and, as shown in the data of thelater explained examples, is superior in fatigue characteristics aftershaping and annealing. Further, the center layer has a low hardness, sohas a good workability and can be shaped with a high dimensionalprecision, so the work efficiency in assembly into the chassis issuperior. Further, the residual strain can be eliminated by annealing ata low temperature of 660° C. or less, the heat treatment cost becomesinexpensive, there is little drop in hardness even with annealing, thefatigue characteristics are superior, etc. There are many advantages.

The invention claimed is:
 1. A method of production of an automobilechassis part superior in fatigue characteristics, the method comprisingproducing a steel pipe of a hot-rolled plate of a steel material havinga composition of, by mass %, C: 0.05 to 0.23%, Si: 0.05 to 1.0%, Mn: 0.3to 2.0%, P: 0.03% or less, S: 0.01% or less, Nb: 0.02 to 0.1%, Mo: 0.2to 0.5%, Sol. Al: 0.01 to 0.05%, N: 0.006% or less, and a balance of Fe,and press forming the steel pipe to give a cross-sectional bendingradius of the plate outer surface of 2 to 5 times a plate thickness,then annealing the steel pipe under conditions giving a temperingparameter λ defined by λ=T(20+log(t)) of 14000 to 19000 (where T is theabsolute temperature, t indicates the time (h), and the temperature riseis 660° C.), such that (Nb, Mo)C precipitates are produced, and aVicker's hardness at a center of the plate thickness is lower than amaximum value of a Vicker's hardness within 0.5 mm of the plate surfaceby 50 to 150 points.
 2. A method of production of an automobile chassispart superior in fatigue characteristics, the method comprisingproducing a steel pipe of a hot-rolled plate of a steel material havinga composition of, by mass %, C: 0.05 to 0.23%, Si: 0.05 to 1.0%, Mn: 0.3to 2.0%, P: 0.03% or less, S: 0.01% or less, Nb: 0.02 to 0.1%, Mo: 0.2to 0.5%, Sol. Al: 0.01 to 0.05%, N: 0.006% or less, and a balance of Feand further including one or more of Ti: 0.005 to 0.03%, V: 0.005 to0.1%, Cr: 0.1 to 0.5%, Cu: 0.001 to 0.5%, Ni: 0.001 to 0.5%, B: 0.0001to 0.003%, Ca: 0.0001 to 0.003%, and Mg: 0.0001 to 0.004%, press formingthe steel pipe to give a cross-sectional bending radius of the outersurface of the plate of 2 to 5 times a plate thickness, then annealingthe steel pipe under conditions giving a tempering parameter λ definedby λ=T(20+log(t)) of 14000 to 19000 (where T is the absolutetemperature, t indicates the time (h), and the temperature rise is 660°C.), such that (Nb, Mo)C precipitates are produced, and a Vicker'shardness at a center of the plate thickness is lower than a maximumvalue of a Vicker's hardness within 0.5 mm of the plate surface by 50 to150 points.